The present invention relates to a thrust hydrodynamic bearing, to a spindle motor equipped with the thrust hydrodynamic bearing, and to a disk drive utilizing the spindle motor, that are capable of eliminating air bubbles that can build up nearby the bearing center.
Spindle motors for driving recording disks such as hard disks include bearing means for supporting axial loads that act on the rotor. Thrust hydrodynamic bearings are conventionally employed as the axial load bearing means. In a fluid such as oil, retained between two axially opposing planar surfaces, thrust hydrodynamic bearings generate dynamic pressure when the rotor rotates. The hydrodynamically generated thrust pressure serves as the axial load bearing pressure.
FIG. 7 depicts a conventional thrust hydrodynamic bearing 1. The dynamic-pressure generating grooves formed in this conventional thrust bearing 1 for generating hydrodynamic pressure are so-called pump-in type spiral grooves 2, which induce radially inward acting dynamic pressure in the oil. By virtue of the spiral grooves, nearby the bearing center a pressure peak where the dynamic pressure becomes quite large appears, while heading radially outward the pressure declines. This pressure peak area in the dynamic bearing supports loads acting on the rotor.
The spiral grooves 2 develop dynamic pressure with comparatively better energy efficiency than herringbone grooves formed by two sets of spiral grooves in combination. In addition, the spiral grooves 2 make diametric reduction of the thrust hydrodynamic bearing 1 possible, which lets the motor be run at low peripheral speeds, and which is a way to decrease bearing losses.
Nevertheless, air bubbles will eventually become present in the area where the spiral grooves 2 are formed. The air bubbles will shift from the high end to the low end along the pressure gradient at which the thrust bearing 1 generates dynamic pressure. Accordingly, the air bubbles shift radially outward, toward where the pressure is lower. Herein, one way to exhaust the air bubbles to the bearing exterior is to arrange a communicating hole in the radially outward area of the bearing, as a communication to the bearing exterior. Because the bearing pressure distribution develops with axial symmetry, however, in the bearing center vicinity there will only be a slight pressure gradient. Despite the communicating hole, therefore, the air bubbles that eventually will be present nearby the center are less likely to be eliminated.
When air bubbles build up like this nearby the center of the bearing under a higher-temperature environment, they increase in volume because the coefficient of thermal expansion of the air bubbles is greater than that of the oil. The expanding air bubbles cause the oil to effuse to the bearing exterior. A similar phenomenon occurs even under a lower-temperature environment. Effusion of oil decreases the amount of oil retained in the bearing. Consequently the rigidity of the bearing declines and moreover the oil reserve depletes prematurely; and other problems, such as deterioration in endurance and degradation in reliability of the bearing, arise.
An object of the present invention is to eliminate air bubbles liable to build up nearby the bearing center, and thereby to yield stabilized axial bearing force, in a thrust hydrodynamic bearing in which dynamic pressure is generated by pump-in type spiral grooves.
Yet another object of the present invention is with a simple structure to eliminate to the bearing exterior air bubbles liable to build up nearby the center of a pump-in type thrust hydrodynamic bearing.
A still further object of the invention is to structure a pump-in type thrust hydrodynamic bearing for superior endurance and reliability of the bearing, and of a spindle motor furnished with the bearing.
An additional object is to provide a disk drive that operates stably over the long term.
In order to achieve the foregoing objects, in a pump-in type thrust hydrodynamic bearing construction according to the present invention an asymnmetrical auxiliary groove is added to spiral grooves formed circularly symmetrical with respect to the axial center. The addition of the asymmetrical auxiliary groove makes the circularly symmetrical pressure distribution created by the spiral grooves asymmetrical. Air bubbles building up nearby the center of the spiral grooves are thereby shifted to the area where the spiral grooves are formed. Around the inside of the area where the spiral grooves are formed, the dynamic pressure of the oil will be high. Air bubbles in a hydraulic fluid tend to shift from a region where the fluid pressure is high to a region where the fluid pressure is low. Owing to this tendency, air bubbles inside the area where the spiral grooves are formed get pushed out beyond the periphery.
This accordingly is a way surely and readily to eliminate air bubbles intermixing with the oil. Thus eliminating air bubbles lets movement of oil in the direction of the bearing center by virtue of the pump-in type spiral grooves occur smoothly, which yields stabilized axial bearing force. In addition, oil leakage from the bearings occurring due to air bubbles is effectively checked, which improves the bearing reliability and endurance.
As far as manufacturing is concerned the spiral grooves and/or the auxiliary groove can by formed by electrochemical machining, cutting, or pressworking processes. Further, the auxiliary groove should be of a shape or configuration that makes the pressure distribution that the spiral grooves develop asymmetric with respect to the axial center. For example, the auxiliary groove may be straight or circularly arcuate, and positioned radially inward of the spiral grooves, asymmetric with respect to the axial center and having a radially outward end from which the auxiliary groove extends towards the bearing center. Alternatively, the auxiliary groove may be formed by extending radially inward a part of the inner edge of the spiral grooves. Furthermore, furnishing in the outer margin of the thrust plate a communication hole that communicates with the bearing exterior is a way to exhaust air bubbles pushed out beyond the periphery of the spiral grooves.
In addition, the present invention can be realized as a spindle motor having a thrust bearing as described above. Spindle motors thus embodied may be employed in spindle motor applications for driving magnetic disks such as hard disks, magneto-optical disks, optical disks such as CD-ROMs and DVDs, and like recording disks. These spindle motors could be used under various environments, particularly in high-temperature, low-pressure environments. Air assimilated into oil under a high-temperature, low-pressure environment is liable to turn into bubbles. Air bubbles generated within the oil are eliminated surely and readily by establishing an auxiliary groove like the foregoing, associated with the thrust bearing spiral grooves. Consequently, problems originating in air bubbles, such as oil effusion and degradation in bearing rigidity, are effectively averted.
From the following detailed description in conjunction with the accompanying drawings, the foregoing and other objects, features, aspects and advantages of the present invention will become readily apparent to those skilled in the art.